The invention relates to a method for controlling a rotational speed of a drive, especially a heavy-duty drive, and to a device working in accordance with the method. The drive drives a shaft and the rotational speed is controlled by means of a control device which is supplied with a control deviation calculated by means of a predetermined or predeterminable desired value for the rotational speed and an actual value for the rotational speed.
Vertical mills for pulverizing brittle materials, for example cement raw material, are sometimes inclined to strong mechanical torsional vibrations in the drive train. To be able to counteract these torsional vibrations using control technology, for controlling a heavy-duty drive driving such a mill, precise information about the current angle of rotation or about its temporal derivations, i.e. for example the rotational speed is required at at least one point of the drive train. The rotor of the motor comes into consideration for example for recording measured values. Conventional angular position encoders are based on a relative movement of a measuring scale being detected by a scanning unit. The scanning unit in such cases is suitably disposed at a fixed location and the measuring scale is attached to the circumference of the shaft for example.
The accuracy of such a measurement, for example a measurement of the rotational speed, suffers however from a relative movement between measuring scale and scanning unit being composed of an overlaying of different movement components. In practice these movement components can be the result of play or other irregularities of the support of the shaft, a flexing of the shaft as result of non-roundness, ovality or a circumferential eccentricity of the shaft as well as installation or manufacturing inaccuracies of the measuring scale.
In controlling the rotational speed of the shaft, in addition to the rotation, all other movement components are to be considered as fault components. An inaccurate measurement signal necessarily leads to a poor quality of control. Specifically when a fault component has a periodicity, for example a fault component resulting from an eccentricity of the measuring scale, this can lead to significant problems. If the periodic fault component with at least one of its spectral line components encounters an inherent frequency of the control system, this can fuel a resonance.
Because vertical mill drives and other heavy-duty drives have as yet not been highly dynamically controlled, this problem has not played any role to date and accordingly, to the best of the inventor's knowledge, has not been investigated.